Add cloneNode and toxml as some pages fail when extracting RDFa

[andrix]

Trying to extract some RDFa from a site, I got AttributeError exception because of the lack of implementation of cloneNode and toxml.

I've added both methods and now it's working.

[codecov-io]

Codecov Report

Merging #38 into master will decrease coverage by 0.55%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage    86.8%   86.25%   -0.56%     
==========================================
  Files           3        3              
  Lines         235      240       +5     
  Branches       47       47              
==========================================
+ Hits          204      207       +3     
- Misses         29       31       +2     
  Partials        2        2

Impacted Files	Coverage Δ
extruct/rdfa.py	74.79% <66.66%> (-0.63%)	⬇️

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[redapple]

Good catch @andrix !
I can reproduce the exceptions with this page for example: http://nielslubberman.nl/drupal/
It's related to 'http://purl.org/rss/1.0/modules/content/encoded' property (with RDFa 1.0 at least)